As it's a PME, I'd just connect the DNO earth up to the MET along with your rod. All you're doing is increasing the 'M' (Multiple) part of the DNO's network. The reason that TNCS isn't allowed on building sites is because of risk of damage to the PEN during heavy construction phases - hence having a rod to 'guarantee'(?!) ADS in the event of a fault.
BS7430 says.. 4.2.3.4 "The TN-C-S system is also known as a 'protective multiple earthed' (PME) system. The PEN conductor, which is referred to as a combined neutral and earth (CNE) conductor, is earthed at the source and extremities of the distribution mains and points in-between hence the reference to multiple earthing. Multiple earthing of the CNE conductor ensures that if the conductor becomes open circuit for any reason, exposed-conductive parts remain connected to earth;...."
So what is happening here in practice is that your rod / TT is making a circuit back to the DNO's nearest earthing point (probably a tape under a pole), along with a gazillion other multiple paths, using the mass of actual planet earth to do so. This 'secures' a return circuit for safety / ADS in the event that the CNE isn't there due to damage (and logically, if the live/s aren't there either in the event of a completely severed cable then it basically doesn't matter....) - this is why you can't use a CNE on a building site, in case it get's damaged. But you can absolutely still use it when backed up with a temporary TT system.